1. Field of Invention
This invention relates to a signal isolating device, utilized, for example, in a signal converter which converts an output signal from a sensor, such as a temperature sensor installed in process control equipment, into a normalized signal; and more particularly, to such a device which utilizes pulse width modulated signals and photocouplers to provide signal isolation.
2. Description of the Prior Art
Signal converters which convert signals obtained from various sensors into normalized signals and isolate signals, are disclosed, for example,in U.S. Pat. No. 4,956,795. In such signal converters, the signals from a sensor are amplified, linearized and then converted into a pulse width modulated signal which is proportional to the input signal from the sensor, by a voltage to pulse width converter circuit. This pulse width modulated signal is isolated by photocouplers, which also transmits the signal, and then the pulse width modulated signal is returned to the original voltage signal by a pulse width to voltage signal converter.
In signal converters of this type, when the output signal from the voltage to pulse width converter circuit is at a high level or a low level, an electrical current of the order of 10 mA continues to flow through the light emitting diodes of the photocouplers. Thus, when the pulse width is greatest, or when the duty factor is greatest, this current continues to flow. Consequently,the entire signal converter, using the prior art signal isolating circuit, consumes a large amount of electrical power. This presents a problem in a two wire signal converter which utilizes electrical power from a signal sent through the two wire transmission path for its operation and which two wire transmission path is also used as a path for transmitting the normalized signal.
Thus, the prior art isolating devices leave much to be improved.